In recent years, there has been an increasing demand for electromagnetic interference (EMI) shielding with the development of advanced electronic devices and communication instruments because high-frequency microwaves generate undesired noise, which can affect the proper operation of commercial, military, and scientific electronic devices as well as the health of our human body. In this study, we investigated the effect of multiwalled carbon nanotube (MWCNT) addition to the Fe-Si-Al alloy (Sendust)/polymer blend on the electromagnetic wave absorption and electromagnetic interference (EMI) shielding. Ternary composites (flaky Fe-Si-Al alloy (Sendust)/MWCNTs/polymer) were fabricated using a twin-screw internal mixer and a roll-milling machine. The flaky Sendust alloy particles were well oriented in the roll-milling direction at a thickness of 1 mm, and MWCNTs were also well dispersed. The addition of MWCNTs increases the dielectric loss of the composite by increasing the interfacial polarizations and dipolar polarizations and generating conductive paths. The reflection loss reached -17 dB at 4.5 GHz with 5 wt % MWCNT addition, but the power loss in the near field rises more rapidly with MWCNT addition. The absorption efficiency of the ternary composite (Sendust/MWCNTs/polymer) was significantly increased compared to the binary composite (Sendust/polymer) due to dielectric property enhancement by MWCNT addition. The total shielding effectiveness (SE) value increased with the amount of MWCNT. The ternary hybrid composites are light but exhibit a high SE in a wide frequency range. Thus, they are appropriate for the production of light and thin-film materials that are suitable for electromagnetic wave absorption and EMI shielding.